(a) Field of the Invention
The present invention relates to a compact high vari-focal ratio zoom lens system and, more particularly, to a compact high vari-focal ratio zoom lens system which comprises four lens groups and which is suitable for a camera with a lens shutter.
(b) Description of the Prior Art
In recent years, due to the fact that full automatization of cameras is being promoted, there is a tendency that variable focal length lens systems are adopted also as photographic lens systems for so-called cameras with lens shutters which are characterized by portability and, consequently, it is required to develop a compact zoom lens system.
A lens system, which is to be used chiefly with a camera with a lens shutter as mentioned in the above, is not subject to a restriction given to lens systems for single-lens reflex cameras, i.e., a restriction to ensure a particular back focal length, and it is therefore possible to make the lens system compact by making the back focal length short.
To make a lens system compact by making the back focal length thereof short as mentioned in the above, it is theoretically known that it is effective when the lens system is arranged to comprise two lens groups to which refractive power is distributed in such manner as a positive lens group and a negative lens group in the order from the object side. For lens systems to be used with a camera with a lens shutter, it is proposed to make the lens system compact by arranging, for example, that a semi-wide angle fixed focal length lens system or a variable focal length lens system of focal length change-over type with two different focal lengths comprises two lens groups with the above-mentioned distribution of refractive power.
Besides, in the field of zoom lens systems, zoom lens systems arranged as described below are already proposed, i.e., a zoom lens system comprising two lens groups with the above-mentioned distribution of refractive power, a zoom lens system comprising three lens groups arranged by developing the above-mentioned zoom lens system comprising two lens groups and, furthermore, a zoom lens system comprising four lens groups arranged so as to attain improved performance, and those known zoom lens systems are actually used with cameras with lens shutters.
However, the above-mentioned known zoom lens systems have a disadvantage that the vari-focal ratio thereof is about 1.5 only and, therefore, they are unsatisfactory from the view point of variation of the field angle. In other words, said known zoom lens systems do not fulfill the requirement of users in general who want to take photographs with variation of perspective. This means that, it is required to increase the vari-focal ratio at least to about 2 or to make it higher than 2. Besides, users of cameras with lens shutters are dissatisfied with the optical performance of lens systems thereof and, therefore, it is necessary to improve the optical peformance.
The zoom lens system disclosed by Japanese Published unexamined patent application No. 128911/81 is known as a typical type of the above-mentioned kind of know zoom lens systems. Said known zoom lens system comprises two lens groups, i.e., a first lens group having positive refractive power and a second lens group having negative refractive power, in the order from the object side and, at the time of zooming, said lens groups are moved as shown in FIG. 1.
Said type of known zoom lens system has such character that it has a telephoto type lens composition in the wide position (W), the principal point is put toward the object side and, therefore, the back focal length is short.
The zoom lens system disclosed by Japanese published unexamined patent application No. 201213/82 is also known as a zoom lens system comprising two lens groups, i.e., a positive lens group and a negative lens group, in the same way as above.
Even in case of the above-mentioned kind of zoom lens systems, it is possible to obtain a compact zoom lens system comprising a small number of lenses as far as the vari-focal ratio is about 1.5. However, when it is attempted to obtain a zoom lens system with a vari-focal ratio about 2, the amount of movement of the second lens group, which serves as a focal length varying lens group, becomes large at the time of zooming and, consequently, positions where rays pass the lens system in the wide position (W) become different from positions where rays pass the lens system in the teleposition (T). Especially, offaxial rays which pass the rearmost lens (the lens arranged on the image side) of the second lens group pass the marginal portion of said lens, and the outer diameters of the lenses become necessarily large. As a result, it becomes difficult to correct chromatic aberration and monochromatic aberrations in well balanced state. Therefore, a zoom lens system comprising two lens groups is not suitable for making the vari-focal ratio high.
The zoom lens system disclosed by Japanese published unexamined patent application No. 137813/83 is known as a zoom lens system comprising three lens groups. However, the above-mentioned known zoom lens system is arranged only by dividing a part of the first or second lens group into two portions and, from the view point of basic composition, said known zoom lens system may be regarded as a zoom lens system comprising two lens groups. The above-mentioned kind of zoom lens system is advantageous for correction of aberrations but does not contribute to increase of the vari-focal ratio.
Known zoom lens systems described so far are arranged to be used with a camera with a lens shutter as mentioned before or to be used with a video camera or the like and are arranged to make the lens system compact by utilizing the fact that it is not necessary to take the restriction related to the back focal length into consideration unlike the case of lens systems for single-lens reflex cameras.
However, even in case of a lens system for a camera with a lens shutter, it is necessaryb to avoid making the focal length short by paying attention only to correction of aberrations when consideration is given to the compactness of the lens system, flare to be caused by detrimental rays and so forth.
Furthermore, in cases of known zoom lens systems described so far, the optical performance is not satisfactory in spite of the fact that the vari-focal ratio is about 1.5 only and, especially, correction of chromatic aberration and curvature of field is not sufficient in most cases. Therefore, most of them do not have allowance to make the vari-focal ratio high.
As described so far, a zoom lens system comprising two lens groups and a zoom lens system comprising three lens groups arranged by developing the above-mentioned zoom lens system comprising two lens groups are not suitable for making the vari-focal ratio high because the maximum amount of movement for zooming required for the focal length varying lens group becomes large, the magnification to be imparted to said lens group becomes high, and it is extremely difficult to attain favourable optical performance over the whole vari-focal range.
When the above-mentioned points are taken into consideration, it may be considered thata zoom lens system can be made compact when the zoom lens system is arranged to comprise four lens groups by distributing refractive power to respective lens groups in such manner as positive, negative, positive and negative in the order from the object side as shown in FIG. 2, in other words, when the zoom lens system is arranged to comprise a first lens group G.sub.1 having positive refractive power, a second lens group G.sub.2 having negative refractive power, a third lens group G.sub.3 having positive refractive power, and a fourth lens group G.sub.4 having negative refractive power, and when said zoom lens system is arranged to move respective lens groups as shown in FIG. 2 at the time of zooming so that the magnifications are thereby imparted to respective lens groups and so that the lens systemis thereby made compact.
The zoom lens system disclosed by Japanese published unexamined patent application No. 57814/85 is known as a zoom lens system comprising four lens groups whose refractive powers are arranged to be positive, negative, positive and negative in the order from the object side. FIG. 3 shows the pattern of movements of respective lens groups of said known zoom lens system at the time of zooming. The zooming method shown in FIG. 3 is already popular in case of zoom lens systems for single-lens reflex cameras. Said zooming method is characterized in that the second lens group is kept fixed at the time of zooming so that the number of movable lens groups becomes three. For said known zoom lens system, it is not necessary to make the back focal length longer than required. Therefore, said known zoom lens system is arranged to have a telephoto type composition in order to make the lens system compact and may be considered that it has such composition that the first lens group of the afore-mentioned known zoom lens system comprising two lens groups is divided into two lens groups. However, in case of said known zoom lens system, the vari-focal ratio is about 1.6 only, and this degree of vari-focal ratio can be attained also by a zoom lens system comprising two lens groups. Moreover, when it is attempted to make the vari-focal ratio higher, the airspace between the second and third lens groups along the optical axis should be made sufficiently wide in the wide position in order to prevent the mechanical interference between the lens groups to be caused in the teleposition because the second lens group is arranged to be kept fixed at the time of zooming and, therefore, it becomes difficult to made the lens system compact.